Superconducting proximity effect of Nb

Previous investigators have studied the proximity effect of a number of nontransition-metal superconductors. In this investigation the proximity effect of Nb (a transition-metal superconductor) has been studied using the Nb-Cu and Nb-Ni systems. The films were sputtered sequentially without breaking vacuum. The sandwiches were maintained at temperatures below 200A°K until after Tc had been measured, to preclude chemical or metallurgical interactions. Sandwich Tc has been obtained as a function of Nb film thickness for thick overlayer films. The Nb-Cu data are in agreement with the de Gennes-Werthamer theory. The Nb-Ni data follow the form of the adjustable-parameter (βa2πkBτs) theory proposed by Werthamer. However, Ni is found to have less effect on Nb than was reported for Pb: a value of β=7 is obtained for Nb compared to β=43 for Pb. Possible explanations of the observed behavior are discussed. © 1968 The American Physical Society.